1. Field of the Invention
The present invention relates to a travel path displaying apparatus in which a current location and a path of travel of a running body, such as an automobile, (hereinafter referred to as a vehicle) are displayed on a display screen which has a map previously displayed thereon and more particularly, it relates to a system for displaying a current location of a running body on a map.
2. Description of the Prior Art
Recently, in an attempt to provide a guidance to a driver of an automobile to prevent the driver from losing his way, during driving for example in a strange place, there has been developed a travel path displaying apparatus, as hereinafter described more in detail, which includes a distance detector for detecting a distance of travel depending upon a running speed of the vehicle and a direction detector for detecting a running direction and an amount of change of direction, from time to time, wherein the current location of the vehicle on a two-dimensional coordinate is successively computed from the respective results of the above-mentioned detections and the location thus computed is memorized and displayed in the form of a constantly changing pattern of continuous point information on a display screen having a map including roads or the like previously displayed thereon, so that the driver may confirm the current location of his vehicle.
In such travel path displaying apparatus, it has been a usual practice to prepare a plurality of area maps which show divided areas, respectively, and to selectively display one of the area maps on the display screen according to the driver's desire. Also, it has been a usual practice to prepare a plurality of area maps of different reduced scales with respect to each of the divided areas and to display the area map having optimum reduced scale on the display screen, depending upon the state of guidance, so that the travel path may be displayed in accordance with the current location calcuated with the corresponding reduced scale.
A usual map is prepared by a method of plane projection, on the basis of lines of longitude and latitude, but a map of larger reduced scale and that of smaller reduced scale are prepared by different methods of projection. Accordingly, it is difficult to treat the maps of larger and smaller reduced scales on a common coordinate system. Furthermore, in a map which is prepared by the method of plane projection on the basis of lines of longitude and latitude, the areas encircled by the respective lines of longitude and latitude are different in size and shape from each other, depending upon the lines of longitude and latitude. Accordingly, it is difficult to treat the respective areas on the common coordinates system.
Particularly, a map having a large reduced scale, such as 1/50,000 or 1/25,000, is prepared on the basis of the UTM coordinates and in case of Japanese map, for example, the map of a whole country is constituted by a plurality of area maps each of which is prepared by the UTM system. In such maps attention is paid only to matching in each of the systems but no attention is paid to the connection between the respective systems. Under the circumstances, at the time of travelling across the connection between the different systems, it is necessary to make conversion of the coordinates for displaying the travel path, in accordance with the changeover from the map based on the former UTM system to that based on the UTM system newly displayed on the display screen, so that complicated processing is required.
A further problem was displaying the current location of a running body, which was obtained by the two-dimensional coordinates as described above, on a map, which was prepared by the method of plane projection on the basis of the lines of longitude and latitude. That is, earth has a spherical surface while a map prepared by the method of projection has a flat surface, so that an error in location is produced on a map, particularly, in case of a map having large reduced scale.
Furthermore, it is noted that the travel path displaying apparatus of this kind has a function of measuring a distance between two points set on the map displayed on the display screen and displaying such distance onto said display screen, and such function occupies an important part of the guidance performed by the travel path displaying apparatus. Under such circumstances, the distance between two points on the map within one area displayed on the screen can be easily calculated by directly obtaining the distance between the points on the same coordinates and then applying arithmetic operation to the distance thus obtained in accordance with the reduced scale of the map, but the distance between the points set on the map which extend over a plurality of areas successively displayed on the display screen cannot be directly calculated if use is made of a map in which there is no matching between the coordinates system in one area and that in the other area.